Anaerobic Bio-Oxidation of MTBE AND 1,2-DCA

VANCE, DAVID, ARCADIS Midland, Texas and MARK LUPO, ARCADIS Houston Texas

    A broad range of hydrocarbons are biodegradeable by oxidation. This includes petroleum hydrocarbons, di- and mono-substituted halogenated hydrocarbons, and other xenobiotic hydrocarbons such as MTBE. The dominant oxidation system under near-surface conditions is based on oxygen. However, other bio-oxidation processes also take place under anaerobic conditions using alternate electron acceptors such as nitrate, ferric iron, or sulfate. These biochemical degradation pathways date to the initial development of microbial life early in the Precambrian making them ubiquitous, but with a high degree of spatial variation. The primary advantage to the use of stimulated anaerobic bio-oxidation for remediation is that the stimulating reagents are water soluble. In many instances these reagents are only reactive in the presence of impacting hydrocarbons, allowing for ready transport without consumption by abiotic or biological side reactions. Which is commonly not the case for oxygen.

    A field scale anaerobic bio-oxidation system has been designed and implemented at a large liquid storage terminal along the Houston Ship Channel. As part of the design process a site wide survey was made of the biochemical conditions and anaerobic bio-oxidation pathways available. A discrete plume of MTBE and a mixed plume of benzene and 1,2 DCA are the focus of a proactive remediation program. Each of the constituent systems were evalatued in two bench scale design studies in which the bio-oxidation systems tested included: oxygen; nitrate; ferric chloride (for ferric iron alone); ferric sulfate (with both components potential electron acceptors); sodium sulfate (for sulfate alone); and humate-catalyzed ferric chloride and ferric sulfate.

    The MTBE and benzene were rapidly degraded by oxygen. MTBE was degraded by humate-catalyzed ferric iron almost as rapidly as with oxygen, however the MTBE daughter product tert-butyl alchohol (TBA) was not degraded with humate-catalyzed ferric iron. Ferric chloride alone degraded MTBE and TBA completely at a rate about 20% that of oxygen. Ferric sulfate, and sulfate also slowly degraded MTBE. The nitrate was completely ineffective. In the case of benzene and 1,2-DCA, the oxygen alone was effective for the degradation of benzene. Oxygen did not stimulate the degradation of 1,2-DCA. All of the anaerobic systems did degrade the 1,2-DCA within 190 days, with the humate-catalyzed ferric chloride the most rapid of the set.

    Galleries of horizontal screened pipes have been installed in trenches beneath newly installed storage tanks to deliver treatment fluids in-situ for remediation of the MTBE plume and field pilot scale applications of chelated ferric iron solutions have recently concluded.